Numerous imaging applications require integrated or active illumination to augment ambient lighting and/or to ensure constant scene illumination irrespective of other illumination sources. This is especially true with imagers having higher frame rates. In many cases, it is necessary or desirable for such illumination sources to be non-visible to the human eye, but detectible by an image sensing device.
A generally employed technique for providing non-visible illumination, as defined by the ability to be detected by the human visual system, is to use wavelengths that are longer than the human eye can detect. Generally, these are referred to as infrared wavelengths. Infrared light comprises light with wavelengths in excess of approximately 700 nm. Particularly useful wavelengths are those from approximately 700 nm through approximately 1100 nm, generally referred to as near-infrared wavelengths. These are wavelengths that can be detected by silicon-based photodetectors such as the photodiodes or photocapacitors that are used in essentially all CMOS and CCD imagers. The pervasive nature of silicon based integrated circuits, and consequently that of silicon based photodetectors and silicon base imager chips, makes the resulting cost of these devices lower than the cost of devices fabricated using other materials and technologies, that might offer other ranges of spectral sensitivity. As a result, it is generally desirable to work within the range of 700 nm through 1100 nm for imaging and detection systems. An alternative to the use of infrared wavelengths is the use of wavelengths shorter than the human eye can detect; that is, wavelengths shorter than approximately 420 nm. These are referred to as ultraviolet wavelengths. Generally, however, sensors capable of detecting such wavelengths are less readily available, and generally more expensive than those that detect light in various portions of the infrared wavelength regime. Additionally, wavelengths shorter than approximately 420 nm become increasingly hazardous to both the human eye and other portions of the human body as the wavelength is reduced further into the ultraviolet regime. Consequently, such short wavelength light is generally not as convenient to use for imager and detector systems. In addition, silicon based devices are generally not sensitive to wavelengths significantly lower than approximately 350 nm, which presents another impediment to the use of such short wavelengths for imaging and detection applications, as compared to systems operating at near-infrared wavelengths.
Eye safety issues limit the maximum irradiance of illumination sources for actively illuminated non-visible imaging applications. Limitations apply for both visible and non-visible illumination sources. These eye-safety limitations represent a major challenge for sensing applications in which ambient lighting varies by a large amount, since an actively illuminated sensing system must be capable of essentially overpowering ambient lighting, such as sun light.
Since the irradiance of actively illuminated systems is limited, this implies that the sensitivity of the detector and or imagers must be appropriately high. Generally, photodetectors, whether they are individual elements or combined in arrays to form imagers, provide at least two physical means to control sensitivity. The fundamental sensitivity of a given detector element is determined by the materials properties of the components of the detector. Beyond that fundamental sensitivity, generally referred to in terms of the detector's quantum efficiency, a given detector can effectively have increased sensitivity if it collects light over a longer period of time, for example if it integrates the collected photo-induced charge. Thus, detection or charge-collection time is one aspect of controlling detector sensitivity. Another aspect relates to the physical geometry of the sensor; that is, the effective aperture of the imaging system or detector—how many photons per square centimeter physically impinge upon the sensing surface. However, if either the detector or objects in the scene are moving, additional constraints are placed upon the amount of time available for charge collection, which may limit the detector sensitivity. On the other hand, as detector areas increase, so does cost. Consequently, increasing sensitivity by arbitrary increases in detector area generally is not acceptable. As a result of these constraints, it is important to optimally match detector sensing opportunities or intervals with the available incident light. Typically, this involves pulsing of the illumination light source or sources.
Typically, arrays of imagers are designed such that the image is sensed or “collected” by the array of individual photodetectors, and then read out, one detector element or pixel at a time. The collection process generally involves some mechanism for enabling each of the photodetector elements to collect light for the same length of time; generally this time is referred to as the integration time. Ideally, every detector element in a given imager array will be collecting light for the same length of time, as described above, and synchronized in time. That is, each photodetection element of the array will start collecting light simultaneously at a first time, and stop collecting light simultaneously at a second time. If the detectors do not all start and stop collecting light at the same time, then objects in the image could be distorted due to relative motion of the imaging array and the scene being imaged. Alternatively, if the light collection time period is not identical for all elements of the imaging array, some areas might appear brighter than other areas simply due to the fact that the corresponding pixels or photodetector elements were allowed to collect light for a longer time, thereby effectively shifting the gray scale of the corresponding portions of the image.
Synchronization of all photodetector elements to both start and stop collecting light at the same times prevents the distortion of collected images due to the motion of objects within the scene. Synchronization of the light collection periods of all photodetector elements also enables efficient use of pulsed illumination sources, such as “flash lamps.” Since all photosensitive elements collect light at the same time, if the flash lamp is discharged during that time period, the potential illuminace of all portions of the scene, as determined by the light collected by individual photodetector elements, will be equal, resulting in the most accurate capture of the scene to be imaged. Pulsed illumination generally enables the most efficient use of a limited optical power budget, since the light (the total amount of which is limited by eye safety requirements) is only being emitted during those time periods in which the sensor is capable of detecting the resultant light reflected from the scene being imaged.
Since the photo-induced charge collected at each photodetector element of an array must generally be read out from an array sequentially one row at a time, and since it is desirable to avoid distortion of the resultant image as described above, there are typically significant portions of time during which any given photodetector element is not collecting light. Illuminating the scene during these periods essentially wastes that portion of the illumination, since it cannot be detected.
In order to maximize the sensitivity of an imager in a given application, it is necessary to maximize the integration time. However, in order to capture the motion of objects within a scene (such as typically occurs for imagers used in automotive imaging applications) generally it is necessary to maximize the frame rate as well. The need to simultaneously maximize both sensitivity and imager frame rate produces a situation in which every pixel in each row of the imager array is either collecting light, or is in the process of transferring the photo-detected signal to the imager chip output. This however eliminates the ability to have all pixels collecting light simultaneously. Thus, a need exists to identify a system and method to simultaneously optimize imager sensitivity and image capture frame rate while minimizing image distortion due to temporal shifts in the photosensitivity periods of different portions of the imaging array.
Some contemporary imager arrays are designed such that the individual pixels are capable of simultaneous photocharge collection and photocharge readout of the previous frame. This requires a complicated pixel structure, entailing more transistors per pixel. Consequently, the pixels are larger for a given photocollection area, and the imager array will cost more with lower sensitivity. The contemporary imagers of this type are referred to as global shutter imagers.